In an incinerator of garbage discharged from the general household, hydrogen chloride (HCl) generates from chlorine-containing substances such as vinyl chloride contained in garbage, and discharged from the incinerator in the form of hydrogen chloride gas contained in an exhaust gas as produced from incineration of garbage. Therefore, the exhaust gas discharged from the garbage incinerator has to be released to the open air after removing hydrogen chloride contained therein to a concentration below a prescribed emission standard value of hydrogen chloride.
The following method using a semi-dry type removing apparatus is conventionally known for the purpose of removing hydrogen chloride contained in an exhaust gas discharged from an incinerator.
FIG. 1 is a conceptual view illustrating a conventional semi-dry type apparatus for removing hydrogen chloride from an exhaust gas discharged from an incinerator. This apparatus 1 basically comprises a reactor 2 for removing hydrogen chloride contained in an exhaust gas discharged from an incinerator, a tank 3 for storing slaked lime in slurry, a pump 5 for supplying slaked lime stored in the tank 3 through a supply pipe 4 into the reactor 2, an atomizer 6 for atomizing the supplied slaked lime into the reactor 2, and an electrostatic dust collector 7 for removing a reaction product entrapped in the exhaust gas discharged from the reactor 2. In FIG. 1, 8 is a controller, added to the removing apparatus 1, for adjusting an amount of supply of slaked lime into the reactor 2.
Using the conventional semi-dry type removing apparatus 1 mentioned above, hydrogen chloride contained in an exhaust gas discharged from an incinerator is practically removed therefrom as follows.
The exhaust gas discharged from the incinerator is cooled to a temperature of from about 280.degree. to about 300.degree. C. through exchange in a boiler or the like, and then continuously introduced from above into the reactor 2. On the other hand, slaked lime in slurry stored in the tank 3 is continuously supplied into the reactor 2 through the supply pipe 4 by means of the pump 5, and is atomized in the reactor 2 by means of the atomizer 6. Hydrogen chloride contained in the exhaust gas discharged from the incinerator comes into contact and reacts with the thus atomized slaked lime in the reactor 2 to form a reaction product in the form of basic calcium chloride (CaCl.sub.2. Ca(OH).sub.2.H.sub.2 O), and removed from the exhaust gas. Removal of hydrogen chloride from the exhaust gas discharged from the incinerator is accomplished by adjusting the amount of supply of slaked lime into the reactor 2 by means of the controller 8 so that the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime agrees to a predetermined target concentration thereof. Control of the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime will be described later. The reaction product produced from the reaction of hydrogen chloride with slaked lime is converted into dried powder under the effect of the heat of the exhaust gas, is accumulated in the lower portion of the reactor 2, and is discharged from the lower end of the reactor 2. The exhaust gas from which hydrogen chloride has thus been removed is introduced from the reactor 2 into the electrostatic dust collector 7, in which the reaction product and dust entrapped in the exhaust gas are removed, and then released to the open air through a stack not shown.
FIG. 2 is a block diagram illustrating a control system of the reaction process of hydrogen chloride with slaked lime, in the conventional method for removing hydrogen chloride from an exhaust gas discharged from an incinerator with the use of a known semidry type removing apparatus. The control system of the reaction process comprises the removing apparatus 1 and the controller 8 shown in FIG. 1. Control of the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime is conducted as shown in FIG. 2. As shown in FIG. 2, the concentration (y) of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime is detected at a position in the downstream of the electrostatic dust collector 7, for example, of the removing apparatus 1. Then a deviation (.DELTA.y) of the thus detected concentration (y) of hydrogen chloride from a predetermined target concentration (y.sub.o) thereof is determined. The thus determined deviation (.DELTA.y) is entered into the controller 8 as an actuating signal. The controller 8 determines a target amount of supply (U) of slaked lime into the reactor 2, which is necessary for reducing the thus determined deviation (.DELTA.y) to zero, through, for example, a PID action (proportion (P)+integration (I)+ differentiation (D) action). The controller 8 adjusts an amount of supply of slaked lime into the reactor 2 so that the amount of supply of slaked lime agrees to the thus determined target amount of supply (U) of slaked lime by changing the revolutions of the pump 5 or altering opening of a flow adjusting valve not shown provided in the middle of the supply pipe 4, whereby the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime is controlled so that the above-mentioned deviation (.DELTA.y) becomes zero, in other words, so that the above-mentioned detected concentration (y) of hydrogen chloride agrees to the predetermined target concentration (y.sub.o) thereof.
In the above-mentioned conventional method, using the known semi-dry type removing apparatus, the reaction process of hydrogen chloride with slaked lime has an important meaning for the control of the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime. FIG. 3 is a graph illustrating an example of the step response between the amount of supply of slaked lime into the reactor 2 and the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime, in the reaction process in the conventional method as described above. In FIG. 3, "INLET CONCENTRATION 0F HCl" expresses the concentration of hydrogen chloride in the exhaust gas at the inlet of the reactor 2, "OUTLET CONCENTRATION OF HCl", the concentration of hydrogen chloride in the exhaust gas at the outlet of the reactor 2, and "AMOUNT OF SUPPLIED SLAKED LIME", the amount of slaked lime supplied into the reactor 2. According to FIG. 3, the step response in the reaction process of hydrogen chloride with slaked lime can be approximated to a "dead time+first order lag" system in terms of the dynamic characteristics. FIG. 4 is a graph illustrating a dead time and a time constant in the reaction process of hydrogen chloride with slaked lime in the conventional method. As is clear from FIG. 4, the reaction process has a dead time of about three minutes and a time constant of about one minute. While these values vary with the size of the removing apparatus 1, the dead time is generally far larger than the time constant.
In the case where the dynamic characteristic of the reaction process has a large value of dead time as above, it is difficult to control the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime as described above, by determining the deviation (.DELTA.y) of the detected concentration (y) of hydrogen chloride remaining in the exhaust gas from the target concentration (y.sub.o) thereof, this being a fact well known in the field of the control theory. When it is attempted to remove hydrogen chloride from the exhaust gas by the conventional method so that the concentration of hydrogen chloride remaining in the exhaust gas stably agrees with the predetermined target concentration thereof, it is imperative to reduce gain in the control system of the reaction process. With a lower gain in the control system of the reaction process, elimination of the deviation (.DELTA.y) requires much time even when the deviation (.DELTA.y) becomes larger, resulting in deterioration of control of the concentration of hydrogen chloride remaining in the exhaust gas. FIG. 5 is a graph illustrating an example of change in the concentration of hydrogen chloride reamining in the exhaust gas after the reaction with slaked lime in the case where hydrogen chloride is removed from the exhaust gas by the conventional method. In FIG. 5, "INLET CONCENTRATION OF HCl" expresses the concentration of hydrogen chloride in the exhaust gas at the inlet of the reactor 2, " OUTLET CONCENTRATION OF HCl", the concentration of hydrogen chloride in the exhaust gas at the outlet of the reactor 2, "OUTLET TARGET CONCENTRATION OF HCl", the predetermined target concentration of hydrogen chloride in the exhaust gas at the outlet of the reactor 2, and "AMOUNT OF SUPPLIED SLAKED LIME", the amount of slaked lime supplied into the reactor 2. In the conventional method, as is clear from FIG. 5, control accuracy of the concentration of hydrogen chloride remaining in the exhaust gas is low, largely varying around the target concentration thereof.
Therefore, for the purpose of removing hydrogen chloride contained in the exhaust gas discharged from the incinerator to a concentration below the prescribed emission standard value of hydrogen chloride, it is necessary for the above-mentioned conventional method to set the target concentration (y.sub.o) of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2 after the reaction with slaked lime to a value sufficiently lower than the prescribed emission standard value of hydrogen chloride to control the concentration of hydrogen chloride remaining in the exhaust gas discharged from the reactor 2. However, this requires an excessive amount of supply of slaked lime into the reactor 2, resulting in not only a higher cost for the removal of hydrogen chloride from the exhaust gas but also a heavier load of slaked lime entrapped in the exhaust gas after the reaction with hydrogen chloride on the electrostatic dust collector 7, thus leading to troubles therein.
Under such circumstances, there is a strong demand for the development of a method for removing hydrogen chloride from an exhaust gas discharged from an incinerator, which permits removal of hydrogen chloride contained in the exhaust gas discharged from the incinerator so that the concentration of hydrogen chloride remaining in the exhaust gas after the reaction with slaked lime stably agrees with a target concentration thereof by taking proper measures against a dead time in the control system of the reaction process of hydrogen chloride with slaked lime, but a method provided with such features has not a yet been proposed.